Recent evidence has demonstrated that endothelial-to-mesenchymal transition (EndMT) may play a significant role in a number of diseases associated with mesenchymal cell/fibroblast proliferation, such as fibrosis and cancer. These studies have further demonstrated that EndMT is particularly prominent during angiogenesis. Although the mechanisms of EndMT during angiogenesis are not well-understood, both transforming growth factor-[unreadable] (TGF[unreadable]) signaling and Snail-mediated transcription have been implicated during EndMT in the setting of cardiac development. Furthermore, the functional role of EndMT during angiogenesis has not been addressed. This form of endothelial plasticity may play a crucial role in organizing the mature vasculature during vasculogenesis and angiogenesis. In order to create and organize new blood vessels, endothelial cells must not only proliferate and migrate, they must also recruit vascular support cells and deposit a new vascular basement membrane. It is therefore conceivable that EndMT-derived cells may contribute to the recruitment of vascular smooth muscle cells/pericytes which help organize an appropriate vessel structure. This proposal is designed to systematically study the contribution of EndMT during angiogenesis, with a focus on the underlying signaling and transcriptional mechanisms. The first aim of this proposal will investigate the role of TGF[unreadable] signaling and its downstream effectors during EndMT. Here, in vivo models of angiogenesis will be studied and various molecular biological approaches will be used to study the signaling mechanisms in vitro. Similar techniques will be used in our second aim in order to determine the transcriptional mechanism associated with EndMT. This aim will specifically investigate the role of Snail as a transcriptional repressor. The third and final aim will evaluate the impact of EndMT-derived cells on the progression of angiogenesis. In this aim, we will use a transgenic mouse that allows EndMT-derived cells to be selectively ablated. Together these aims will begin to lay the groundwork for understanding the role of EndMT in disease. Preliminary evidence suggests that the mechanisms of EndMT are similar in various disease states, and therefore the results of this study will likely be relevant to EndMT in a much broader context.